Earlier studies in this laboratory have employed a Bacillus subtilis transfection system to study DNA damage and its repair. The results indicate that: 1) the sensitivity of DNA to biological inactivation is altered in the presence of some proteins and 2) transfection in the presence of low doses of alkylating agents can increase transfection efficiency. The transfection system provides a powerful technique for studying DNA damage and its repair. With this approach it is possible to limit damage specifically to the DNA of interest and to introduce a fixed amount of damage into the DNA before it is used to transfect competent host cells. Therefore it is possible to correlate physical measurements of the numbers of lesions formed in the DNA after a particular treatment with the biological effects of that treatment. We will use the transfection system together with physical measurements of the strength of the DNA-protein association and of the numbers of specific types of lesions to determine: 1) the extent to which proteins associated with DNA alter the numbers of lesions produced by a particular UV dose, 2) how tertiary structure of DNA affects its sensitivity to UV, and 3) whether there are "hot spots" for UV damage. The transfection system will also be used to study chemical carcinogen damage and repair of this damage. Stimulated recombination among damaged DNA strands and inducible repair of DNA damage will be studied for carcinogens with different mechanisms of action. An understanding of these effects is of particular importance, since an error in recombination is one likely mechanism for cellular transformation. With this transfection system, the types of lesions serving as stimuli for enhanced recombination will be determined and a quantitative relationship between the amount of DNA damage and the biological effects of this damage will be obtained.